1. Field of the Invention
The invention relates in general to position measurement apparatus, and more specifically to apparatus for accurately measuring the position of a movable body, such an elevator car, along a travel path.
2. Description of the Prior Art
THE INVENTION RELATES BROADLY TO APPARATUS FOR ACCURATELY MEASURING THE POSITION OF A MOVABLE BODY ALONG A TRAVEL PATH. The invention is particularly well suited to measuring the position of an elevator car along its travel path, and it will be described in this context.
Electromechanical elevator control systems conventionally determine the position of the elevator car with an electromechanical device which is, in effect, a scaled down version of the associated elevator system. The car of the scaled model is driven in synchronism with the movement of the elevator car. An important advantage of the electromechanical floor selector, with it mechanical memory of floor position, is its retentive feature. If the electrical power should fail, the mechanical memory retains car position information. While the electromechanical device provides excellent results, it requires periodic maintenance due to the wear of its moving parts. Further, in order to provide the accuracy necessary to control high speed, high rise elevator systems, a relative large and therefore costly scaled model is required.
Solid state elevator control systems conventionally memorize car position in a binary counter. The binary count may be obtained by counting pulses in an up/down counter, with a pulse being produced for each predetermined increment of car travel away from and back to a reference floor. Examples of systems which use counters and pulses are disclosed in U.S. Pat. Nos. 3,370,676, 3,425,515, 3,526,300, 3,589,474, 3,750,850, 3,773,146 and 3,777,855, and Canada Pat. No. 785,967.
Another arrangement for obtaining a binary count is to drive an encoder in synchronism with car movement. Examples of systems which use an encoder are disclosed in U.S. Pat. Nos. 3,590,335 and 3,743,055.
Still another arrangement for obtaining a binary count is to use coded perforated tape which is driven in synchronism with the elevator car and read by a stationary reader, or by using a stationary coded perforated tape which is read by a reader carried by the elevator car. U.S. Pat. Nos. 1,937,917 and RE 27,185 disclose the use of perforated tape and readers.
Systems which utilize a binary counter as a memory to retain a count indicating car position, lose track of the car in the event of power failure, as the content of the memory is destroyed. When electrical power returns, the memory is reinitialized by moving the elevator car to some preassigned known position, such as the bottom floor.
It would be desirable to provide new and improved position measurement apparatus for an elevator car which develops a count or address indicative of car position in the hoistway, and which regains car position information following a power failure without having to move the car to some preassigned floor. Further, it would be desirable to provide a system with this advantage which uses coded tape capable of determining car position to within about 0.5 inch, or less without resorting to a wide tape having a large plurality of parallel channels. For example, a tape having 15 parallel channels and a tape reader having 15 reader circuits would provide over 32,000 discrete positions in the hoistway, but a tape of this nature would not be practical.